1. Field of the Invention
The present invention relates to a distance measurement apparatus; and, in particular, to a pulsed light type distance measurement apparatus which emits pulsed light toward a target to be measured and determines the distance to the target according to the time elapsed from the emission of light until the receiving the pulsed light reflected by the target.
2. Related Background Art
There has been known a pulsed light type distance measurement apparatus which emits pulsed light toward a target to be measured, receives the pulsed light reflected by the target, and determines the distance to the target according to the time elapsed from the emission of light until the receiving thereof.
FIGS. 13A to 13H show an example of timing charts for measurement operations during the time from the emission to receiving in such a pulsed light type distance measurement apparatus. The operations of the measurement apparatus are controlled according to the reference clock shown in FIG. 13A. At the point in time where the measurement start signal S50 shown in FIG. 13B, which also acts as an emission trigger signal, is switched from Low level to High level, an emission pulse L50 is emitted toward a target as shown in FIG. 13C. The light reflected by the target is received and is converted into an electric signal by a photoelectric converter, so as to yield a light-receiving signal S52 shown in FIG. 13D. Then generated is a measurement stop signal S53 which switches from Low level to High level at the peak point of the light-receiving signal S52. The time difference T between the rising of the measurement start signal S50 and the rising of the measurement stop signal S53 corresponds to the time difference between the emission and receiving.
For measuring the time T with an accuracy higher than the cycle of the reference clock, there has been known a technique in which a fractional time Ta (see FIG. 13F) corresponding to the time elapsed from the rising of measurement start signal S50 until the rising of the second reference clock therefrom, for example, and a fractional time Tb (see FIG. 13G) corresponding to the time elapsed from the rising of measurement stop signal S53 until the rising of the second reference clock therefrom, for example, are determined by a time/voltage converter, and the reference clock number N (see FIG. 13H) from the end of Ta to the end of Tb is counted.